<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  <style>
    div{
      height: 80px;
      width: 100%;
      background-color: pink;
      position: fixed;
      top: 0px;
    }
  </style>
</head>
<body style="height: 2000px;">
  <div class="top-nav">
    顶部导航
  </div>
</body>
</html>